A ceramic composite is a heterogeneous material or article comprising a ceramic matrix and filler such as ceramic particles, fibers or whiskers, which are intimately combined to achieve desired properties. These composites are produced by such conventional methods as hot pressing, cold pressing and firing, hot isostatic pressing, and the like. However, these composites typically do not exhibit a sufficiently high fracture toughness to allow for use in very high stress environments such as those encountered by gas turbine engine blades.
A novel and useful method for producing self-supporting ceramic composites by the directed oxidation of a molten precursor metal is disclosed in copending and Commonly Owned U.S. patent application Ser. No. 819,397, described below in greater detail. However, the processing environment is relatively severe, and there is a need, therefore, to protect certain fillers from the strong oxidation environment. Also, certain fillers may be reduced at least partially by molten metal, and therefore, it may be desirable to protect the filler from this local reducing environment. Still further, the protective means should be conducive to the metal oxidation process, yet not degrade the properties of the resulting composite, and even more desirably provide enhancement to the properties.
It is known in the art that certain types of ceramic fillers serve as reinforcing materials for ceramic composites, and the selection or choice of fillers can influence the mechanical properties of the composite. For example, the fracture toughness of the composite can be increased by incorporating certain high strength filler materials, such as fibers or whiskers, into the ceramic matrix. When a fracture initiates in the matrix, the filler debonds from the matrix and spans the fracture, thereby resisting or impeding the progress of the fracture through the matrix. Upon the application of additional stress, the fracture propagates through the matrix, and the filler begins to fracture in a plane different from that of the matrix, pulling out of the matrix and absorbing energy in the process. Pull-out is believed to increase certain mechanical properties such as work-of-fracture by releasing the stored elastic strain energy in a controlled manner through friction generated between the material and the surrounding matrix.
Debonding and pull-out have been achieved in the prior art by applying a suitable coating to the ceramic filler material. The coating is selected so as to have a lower bonding strength with the surrounding matrix than the filler, per se, would have with the matrix. For example, a boron nitride coating on silicon carbide fibers has been found to be useful to enhance pull-out of the fibers. However, the use of boron nitride coated fibers in composites presents significant processing disadvantages. For example, the production of ceramic matrix composites containing boron nitride coated materials requires the use of reducing atmospheres since a thin layer of boron nitride readily oxidizes at temperatures above 800.degree.-900.degree. C. A reducing atmosphere, however, is not compatible with the directed oxidation of molten precursor metal for fabricating ceramic composites. Further, in the directed oxidation process the coating desirably is compatible with the molten metal in that the molten metal wets the coated filler under the process conditions, for otherwise the oxidation process and matrix growth may be impeded by the filler.
Also, in order to prevent or minimize filler degradation, certain limits may be imposed on the conventional fabrication processes, such as using low processing temperatures or short times at processing temperature. For example, certain fillers may react with the matrix of the composite above a certain temperature. Coatings have been utilized to overcome degradation, but as explained above, the coating can limit the choice of processing conditions. In addition, the coating must be compatible with the filler and with the ceramic matrix.
A need therefore exists to provide coated ceramic filler materials which are capable of debonding and pull-out from a surrounding ceramic matrix. A further need exists to provide coated ceramic filler materials which may be incorporated into the ceramic matrix at elevated temperatures under oxidizing conditions to provide composites exhibiting improved mechanical properties such as increased fracture toughness.
In order to meet one or more of these needs, the prior art shows filler materials bearing one or more coatings. Carbon is a useful reinforcing filler but typically is reactive with the matrix material. It therefore is well known in the art to provide the carbon fibers with a protective coating. U.S. Pat. No. 4,397,901 teaches first coating carbon fibers with carbon as by chemical vapor deposition, and then with a reaction-formed coating of a metallic carbide, oxide, or nitride. Due to a mismatch in thermal expansion between the fiber and the coating, the fiber is capable of moving relative to the coating to relieve stress. A duplex coating on carbon fibers is taught by U.S. Pat. No. 4,405,685. The coating comprises a first or inner coating of a mixture of carbon and a metal carbide and then an outer coating of a metal carbide. The outer coatings prevent degradation of the fiber due to reaction of unprotected fiber with the matrix material, and the inner coating inhibits the propagation of cracks initiated in the outer layer. U.S. Pat. No. 3,811,920, relating to metal matrix composites, discloses coated fibers as a reinforcing filler, such as boron filaments having a silicon carbide surface layer and an additional outer coating of titanium carbide. This reference teaches that the additional coating of titanium carbide improves oxidation resistance as well as provides a diffusion barrier between the filament and metal matrix.
However, the prior art fails to teach or suggest filler materials with a duplex coating for protection from and compatibility with a molten metal in an oxidizing environment during manufacture of the ceramic matrix composite by directed oxidation, and yet in the composite exhibit debonding and pull-out from the surrounding matrix.